Now imagine having a card that was undercosted, did the same effect and gave it lifelink. Wow, that's a saturated card waiting to be abused. Almost-instant speed and a 4-cost 4/4 lifelink is pretty much first pick in draft. It might single-handedly win in Limited actually.

4/5 Constructed
5/5 Limited

MUCard:
Mental Leash - 3UU
Enchantment
When Mental Leash enters the battlefield, you may have it enter with three or four mental counters.
If Mental Leash has three mental counters or less, spells with equal to or less than converted mana cost X can’t be cast, where X is the number of mental counters on Mental Leash.
If Mental Leash has four or more mental counters, spells with equal to or more than converted mana cost X can’t be cast, where X is the number of mental counters on Mental Leash.

It's a little bit wordy but effectively stops your choices between 3 or less CMC spells, or 4 or more CMC spells.